Capacitive Microphone With Integrated Cavity

ABSTRACT

A capacitive microphone and method of fabricating the same are provided. One or more holes can be formed in a first printed circuit board (PCB). A diaphragm can be surface micro-machined onto an interior surface of the first PCB at a region having the one or more holes. Interface electronics can also be interconnected to the interior surface of the PCB. One or more spacer PCBs can be attached to a second PCB to the first PCB, such that appropriate interconnections between interconnect vias are made. The second PCB and first PCB with spacers in between can be attached so as to create a cavity in which the diaphragm and interface electronics are located.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Application Ser. No.60/926,307, filed Apr. 25, 2007, which is hereby incorporated byreference herein in its entirety, including any figures, tables, ordrawings.

BACKGROUND OF INVENTION

Many consumer electronic products incorporate a microphone. As this is ahigh volume market, average selling price (ASP) is typically a keyfactor. The “Yole Silicon Microphone Market Report 2005” [1] projectsthe total silicon microphone market to be 2.75 M units and 221 M$ by2008 with the major applications being mobile phones, PDAs, laptops,PCs, hearing aids, acoustic noise control and automotive crashdetection. Many of these products use conventional electret condensermicrophones (ECM) produced by a number of low cost suppliers. To date,one silicon micromachined microphone (Knowles acoustics SiSonic [2]) hasbeen able to compete effectively in this market by meeting theperformance, reliability, and price expectations set by the ECMsuppliers. Typically, die shrinkage (more parts per wafer) andelimination of processing steps (lower cost per fabrication lot run) canlower the ASP. However, the high costs associated with siliconmicrofabrication currently limits the microfabrication-based costcutting measures. The packaging costs of the device can also be adominant factor. For the SiSonic microphone, the packaging structureincludes a base, a wall, and a lid all made from FR4 printed-circuitboard (PCB) material and laminated together [2]. This package must belarge enough to fit the silicon microphone and amplifier die, as well asthe associated passives.

BRIEF SUMMARY OF INVENTION

Embodiments of the subject invention relate to a method of fabricating acapacitive microphone. Embodiments also pertain to a capacitivemicrophone. In an embodiment, the subject capacitive microphone can usePCB-fabrication technology to realize a low-cost microphone integratedwith the microphone package. FIG. 1 shows a cross-section of a specificembodiment of a capacitive microphone in accordance with the subjectinvention. The embodiment of FIG. 1 is a condenser microphone in which asurface micromachined diaphragm 10 is separated from a porous backplate11 on the interior surface of a sheet of a PCB substrate 12. Thediaphragm 10 can be positioned on the PCB substrate 12 such that a frontsurface of the diaphragm can be exposed to air passing through one ormore apertures 13 formed in the PCB substrate 12. The microphone in FIG.1 can also be an electret microphone by placing permanent electriccharge on either the back plate or the diaphragm and creating an outputsignal based on a change in voltage across the back plate and thediaphragm.

Embodiments of the subject capacitive microphone can be condenser orelectret condenser. In a specific embodiment fabricated using PCB-basedtechnology, a 24″×24″ substrate is utilized, which can save costs forhigh volume. In an embodiment locating the interface electronics withinthe cavity, a conductive interior surface on the enclosure top, wall, orbottom can be used to connect the backplate to the interfaceelectronics, and connecting to the diaphragm, such that no wire bondsare needed. Reducing the need for wire bonds can reduce costs andimprove reliability. In an embodiment, the exterior of the package canbe metal-plated and grounded to shield against electromagneticinterference. In accordance with various embodiments of the invention,lower fabrication cost and an integrated package can allow themicrophone diaphragm to be much larger. The larger diaphragm can improvesensitivity, increase the sensor capacitance, and reduce the noisefloor, resulting in superior performance. Embodiments of the inventioncan incorporate a large back volume such that the microphone can reducecavity stiffening effects with respect to silicon devices that arelimited to a silicon wafer thickness resulting in improved deviceperformance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional schematic of a PCB-based capacitivemicrophone according to an embodiment of the present invention.

FIG. 2 shows a cross-sectional schematic of a PCB-based capacitivemicrophone according to an embodiment of the present invention.

FIG. 3 shows a cross-sectional representation of a PCB board accordingto an embodiment of the present invention.

FIGS. 4A and 4B show a cross-sectional view and top view, respectively,of drilled holes fabricated according to an embodiment of the presentinvention.

FIGS. 5A-5D show cross-sectional schematics illustrating a method offabricating a microphone according to an embodiment of the presentinvention.

FIGS. 6A-6C show cross-sectional schematics illustrating a method offabrication a microphone according to an embodiment of the presentinvention.

DETAILED DISCLOSURE

Embodiments of the subject invention relate to a method of fabricating acapacitive microphone. Embodiments also pertain to a capacitivemicrophone. In an embodiment, the subject capacitive microphone can usePCB-fabrication technology to realize a low-cost microphone integratedwith the microphone package. FIG. 1 shows a cross-section of a specificembodiment of a capacitive microphone in accordance with the subjectinvention. The embodiment of FIG. 1 is a condenser microphone in which asurface micromachined diaphragm 10 is separated from a porous backplate11 on the interior surface of a sheet of a PCB substrate 12. Thediaphragm 10 can be positioned on the PCB substrate 12 such that a frontsurface of the diaphragm can be exposed to air passing through one ormore apertures 13 formed in the PCB substrate 12. The microphone in FIG.1 can also be an electret microphone by placing permanent electriccharge on either the back plate or the diaphragm and creating an outputsignal based on a change in voltage across the back plate and thediaphragm.

Referring to FIG. 1, the two terminals 14 a and 14 b of the condenserare attached to the appropriate surface mounted interface electronics 15(i.e., charge pump, buffer amplifier, etc.) via PCB leads. The interfaceelectronics 15 can be located interior to the PCB package microphone. Ina specific embodiment, a capillary vent hole can be created through thepackage, e.g., through the top or sides, to allow air to flow in and outof the portion of the cavity 16 in contact with the surface of diaphragm10 opposite the surface of the diaphragm 10 toward the one or moreapertures 13, in order to equilibrate pressure within the cavity 16. Thevent allows cavity 16 to be in high acoustic impedance contact with theambient acoustic environment outside of the device. In an alternativeembodiment, venting can be accomplished via apertures in region 17 or,for embodiments utilizing an insulating spacer between the conductivematerial connected to the backplate 11 and the diaphragm 10, theapertures can be in the insulating spacer. Such venting can be providedso as to allow a meaningful signal response below about 10 Hz bymaintaining cavity in high acoustic impedance with the ambient acousticenvironment. A middle PCB layer 18 can be hollowed out to form a spacerlayer between the top PCB layer 12 and bottom PCB layer 19, such that acavity 16 is formed. The PCB layers 18 and 19 can include standardconductive vias 20 to the backside of the bottom PCB 19 and appropriatesolder bumps and/or leads 21 to enable bump-bond/re-flow assembly.

An optional dust cover 22 can be provided over the one or more apertures13. The dust cover 22 can be in the form of a protective mesh. In aspecific embodiment, the dust cover 22 can be a felt top.

FIG. 2 shows a cross-section of another specific embodiment of acapacitive microphone in accordance with the subject invention. Theembodiment of FIG. 2 is a condenser microphone in which a diaphragm 30is separated from a backplate 31 on the interior surface of a sheet of aPCB substrate 32 such that a back surface of the diaphragm 30 can beexposed to air passing through one or more apertures 33 formed inanother PCB substrate 34. Interface electronics 35 can be mounted on thePCB substrate 32. The interface electronics 35 can be located interiorto the PCB package microphone. In one embodiment, a middle PCB layer 36can be hollowed out to form a spacer layer between the top PCB layer 34and bottom PCB layer 32, such that a cavity 37 is formed. The PCB layer32 can include standard conductive vias 38 to the backside of the bottomPCB 32. In a further embodiment, an optional dust cover 39 can beprovided over the one or more apertures 33. The microphone of FIG. 2 canalso be an electret microphone by placing permanent electric charge oneither the back plate or the diaphragm and creating an output signalbased on a change in voltage across the back plate and the diaphragm.

Embodiments of the subject capacitive microphone can be condenser orelectret condenser. In a specific embodiment fabricated using PCB-basedtechnology, a 24″×24″ substrate is utilized, which can save costs forhigh volume. In an embodiment locating the interface electronics withinthe cavity, a conductive interior surface on the enclosure top, wall, orbottom can be used to connect the backplate to the interfaceelectronics, and connecting to the diaphragm, such that no wire bondsare needed. Reducing the need for wire bonds can reduce costs andimprove reliability. In an embodiment, the exterior of the package canbe metal-plated and grounded to shield against electromagneticinterference. In accordance with various embodiments of the invention,lower fabrication cost and an integrated package can allow themicrophone diaphragm to be much larger. The larger diaphragm can improvesensitivity, increase the sensor capacitance, and reduce the noisefloor, resulting in superior performance Embodiments of the inventioncan incorporate a large back volume such that the microphone can reducecavity stiffening effects with respect to silicon devices that arelimited to a silicon wafer thickness resulting in improved deviceperformance.

In a condenser embodiment of the subject microphone, the microphone canwithstand higher operating temperatures and can withstand lead-freesolder re-flow cycles (e.g., around 400° C.), which is a productassembly advantage over ECMs. The enclosure of various embodiments ofthe invention can use a variety of materials, including as examplesprinted circuit board (PCB) or printed wiring board (PWB). PCB and PWBtechnology refer to modern circuit board construction. These boards caninclude multiple laminated dielectric and conductive layers. Thedielectric layer can serve as the structural support. FR4(flame-retardant 4) can be used as the dielectric layer in the boards.Other options include, but are not limited to, FR2, polyimide (forflexible circuits), Getek, Thermount, and Rogers 4050, Rogers 4003 (RFcircuits), etc. The conductive layers (e.g., copper or other metal) canbe etched or “patterned” to provide discrete electrical connectionsbetween various regions of the board.

In an embodiment, surface-micromachining can be used to form amicrophone directly on the board substrate forming a portion of theenclosure. Referring to FIG. 3, a first step can involve selecting a PCB200 having an outer conductor layer 201 on one or both exterior surfacesand one or more dielectric layers 202 in the middle. The dielectriclayer(s) 202 can be fiber glass and the conductor layer 201 can be ametal. Referring to FIGS. 4A and 4B, where FIG. 4A shows across-sectional view and FIG. 4B shows a top view, one or more apertures203 can be created through the PCB board 200 to allow air to travelthrough the PCB. In one embodiment, the one or more apertures 203 can beformed by drilling into the PCB 200. Other techniques for creating theapertures can also be used. In a variety of embodiments, the position,shape and pattern of the one or more apertures 203 can be formed asdesired.

In one embodiment, patterns can be etched in the outer conductorlayer(s) 201 of the PCB board 200 in preparation of interconnecting theelectronics of the microphone. FIG. 5A shows a specific embodiment of aPCB 200A having patterns etched into the conductor layer at one surfaceof the PCB 200A. The patterns can include diaphragm lead lines 201A, abackplate pattern 201B, and interface electronics lead lines 201C.

Referring to FIG. 5B, a diaphragm 204 can then be surface micromachinedonto the interior surface of the PCB 200A. In addition, the interfaceelectronics 205, which can be provided in the form of an ApplicationSpecific Integrated Circuit (ASIC), can be interconnected to theinterior surface of the PCB 200A either before or after micromachiningthe diaphragm 204 onto the PCB surface. Referring to FIG. 5C, one ormore spacer PCBs 206 can be attached to a bottom PCB 207, or to the topPCB 200A, such that appropriate interconnections between interconnectvias 208 are made. Referring to FIG. 5D, the bottom PCB 207 and top PCB200 with spacers 206 in between can be attached so as to create a cavity209 in which the diaphragm 204 and ASIC interface electronics 205 arepositioned. It is understood that other techniques can be used to createthe cavity.

In another embodiment as illustrated in FIGS. 6A-6C, a diaphragm can belocated on a second PCB that is not provided with one or more apertures.Referring to FIG. 6A, patterns can be etched in the outer conductorlayer(s) of a PCB substrate 207A in preparation of interconnecting theelectronics of the microphone. The patterns can include diaphragm leadlines 201D, a backplate pattern 201E, and interface electronics leadlines 201F. Interconnect vias can be formed in the PCB substrate 207A toprovide electrical connections to the outside.

Referring to FIG. 6B, a diaphragm 204 can then be surface micromachinedonto the interior surface of the PCB 207A. In addition, the interfaceelectronics 205, which can be provided in the form of an ApplicationSpecific Integrated Circuit (ASIC), can be interconnected to theinterior surface of the PCB 207A either before or after micromachiningthe diaphragm 204 onto the PCB surface. Referring to FIG. 6C, one ormore spacer PCBs 206 can be attached to the PCB 207A or to a top PCB 200having one or more apertures 203. Then, the bottom PCB 207A and top PCB200 with spacers 206 in between can be attached so as to create a cavity209 in which the diaphragm 204 and ASIC interface electronics 205 arepositioned. It is understood that other techniques can also be used tocreate the cavity. When constructing a complete package with the ASICinterface electronics and, optionally other components embedded, thesystem can be a multi-chip module (MCM), where MCM technology refers toassembling one or more devices, chips, or components on a commonsubstrate to form a more complex system. MCMs can be further classifiedby the supporting technology used to form the electricalinterconnections on the substrate. Embodiments of the invention canutilize MCM-L, MCM-D, and/or MCM-C, where MCM-L (laminated MCM) involvesa base substrate that is a multi-layer laminated PCB, MCM-D (depositedMCM) involves a base substrate that is often a semiconductor wafer withfilms deposited using thin film deposition techniques, and MCM-C(ceramic substrate MCM) involves a base substrate that is laminatedceramic board, (e.g. low-temperature co-fired ceramic (LTCC)) most oftenused for RF circuits.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

REFERENCES

-   [1] “SIMM '05, Yole Silicon Microphone Market Report 2005 Technology    and Market Analysis: From Silicon Microphone Device to Microphone    Modules”, Yole Development, September 2005.-   [2] P. V. Loeppert and S. B. Lee, “SiSonic™—The first commercialized    MEMS microphone,” in Proceedings of Solid-State Sensor and Actuator    Workshop, Hilton Head Island, S.C., 2006, pp. 27-30.

1. A microphone, comprising: a back plate positioned on a portion of aprinted circuit board; and a diaphragm positioned relative to theportion of the printed circuit board so as to form a first volumebetween the diaphragm and the back plate, wherein a front surface of thediaphragm is in contact with the first volume, wherein a back surface ofthe diaphragm is in contact with a second volume, wherein one of thefirst volume and the second volume is in low acoustic impedance contactwith the ambient acoustic environment and the other of the first volumeand the second volume is in high acoustical impedance contact with theambient acoustic environment, wherein when the diaphragm flexes inresponse to acoustic pressure, an output signal related to the acousticpressure on the diaphragm is produced.
 2. The microphone according toclaim 1, wherein the diaphragm comprises a conducting material, whereinthe back plate comprises a conducting material, wherein when a biasvoltage is applied between the diaphragm and the back plate, flexing ofthe diaphragm causes a change in electric charge across the diaphragmand the back plate, wherein the output signal is produced from thechange in electric charge across the diaphragm and the back plate. 3.The microphone according to claim 1, wherein the diaphragm comprises apermanent electric charge, wherein the back plate comprises a conductingmaterial, wherein flexing of the diaphragm causes a voltage changeacross the diaphragm and the back plate, wherein the output signal isproduced from the voltage change across the diaphragm and the backplate.
 4. The microphone according to claim 1, wherein the diaphragmcomprises a conducting material, wherein the back plate comprises apermanent electric charge, wherein flexing of the diaphragm causes avoltage change across the diaphragm and the back plate, wherein theoutput signal is produced from the voltage change across the diaphragmand the back plate.
 5. The microphone according to claim 1, wherein thefirst volume is in low acoustic impedance contact with the ambientacoustic environment and the second volume is in high acoustic impedancecontact with the ambient acoustic environment.
 6. The microphoneaccording to claim 1, wherein the first volume is in high acousticimpedance contact with the ambient acoustic environment the secondvolume is in low acoustic impedance contact with the ambient acousticenvironment.
 7. The microphone according to claim 5, further comprising:an enclosure, wherein the second volume is within the enclosure.
 8. Themicrophone according to claim 6, further comprising: an enclosure,wherein the first volume is within the enclosure.
 9. The microphoneaccording to claim 7, wherein the enclosure comprises: a printed circuitboard; a bottom printed circuit board; one or more spacer printedcircuit boards, wherein the printed circuit board, the one or morespacer printed circuit boards, and the bottom printed circuit board areattached to form the enclosure.
 10. The microphone according to claim 8,wherein the enclosure comprises: the printed circuit board; a bottomprinted circuit board; one or more spacer printed circuit boards,wherein the printed circuit board, the one or more spacer printedcircuit boards, and the bottom printed circuit board are attached toform the enclosure.
 11. The microphone according to claim 9, wherein thediaphragm is within the enclosure.
 12. The microphone according to claim10, wherein the diaphragm is within the enclosure.
 13. The microphoneaccording to claim 9, wherein the second volume is formed by the printedcircuit board, the diaphragm, the one or more spacer printed circuitboards, and the bottom printed circuit board.
 14. The microphoneaccording to claim 10, wherein the first volume is formed by thediaphragm and the back plate.
 15. The microphone according to claim 5,further comprising one or more apertures through the printed circuitboard that allow acoustic waves to pass through the one or moreapertures from the ambient acoustic environment into the first volumeand from the first volume to the ambient acoustic environment.
 16. Themicrophone according to claim 6, further comprising one or moreapertures through the printed circuit board that allow acoustic waves topass through the one or more apertures from the ambient acousticenvironment into the second volume and from the second volume to theambient acoustic environment.
 17. The microphone according to claim 2,further comprising a means for applying a bias voltage between the backplate and the diaphragm.
 18. The microphone according to claim 17,wherein the means for applying a bias voltage between the back plate andthe diaphragm comprises interface electronics.
 19. The microphoneaccording to claim 17, further comprising an electrical impedance bufferamplifier that produces the output signal related to the acousticpressure on the diaphragm.
 20. The microphone according to claim 19,wherein the output signal related to the acoustic pressure on thediaphragm is a low impedance output signal.
 21. The microphone accordingto claim 1, wherein the diaphragm is micromachined on the printedcircuit board.
 22. The microphone according to claim 21, wherein thediaphragm is grown or deposited onto the printed circuit board.
 23. Themicrophone according to claim 1, wherein the diaphragm is placed on theprinted circuit board.
 24. The microphone according to claim 5, furthercomprising a vent passing through the enclosure that allows ambientfluid to pass back and forth between the second volume and the ambientacoustic environment while maintaining the second volume in highacoustic impedance with the ambient acoustic environment.
 25. Themicrophone according to claim 24, wherein the ambient fluid is air. 26.The microphone according to claim 6, further comprising a vent passingthrough the enclosure that allows ambient fluid to pass back and forthbetween the first volume and the ambient acoustic environment whilemaintaining the first volume in high acoustic impedance with ambientacoustic environment.
 27. The microphone according to claim 26, whereinthe ambient fluid is air.
 28. The microphone according to claim 5,further comprising: a vent to allow ambient fluid to pass back and forthbetween the first volume and the second volume while maintaining thesecond volume in high acoustic impedance with the ambient acousticenvironment.
 29. The microphone according to claim 28, wherein theambient fluid is air.
 30. The microphone according to claim 6, furthercomprising: a vent to allow ambient fluid to pass back and forth betweenthe second volume and the first volume while maintaining the firstvolume in high acoustic impedance with the ambient acoustic environment.31. The microphone according to claim 30, wherein the ambient fluid isair.
 32. A method of fabricating a microphone, comprising: providing abackplate on a portion of a printed circuit board; locating a diaphragmrelative to the portion of the printed circuit board so as to form afirst volume between the diaphragm and the back plate, wherein a frontsurface of the diaphragm is in contact with the first volume, wherein aback surface is in contact with a second volume, wherein one of thefirst volume and the second volume is in low acoustic impedance contactwith the ambient acoustic environment, and the other of the first volumeand second volume is in high acoustical impedance contact with theambient acoustic environment, wherein the diaphragm flexes in responseto acoustic pressure; and producing an output signal related to theacoustic pressure on the diaphragm.
 33. The method according to claim32, wherein providing the backplate on the portion of the printedcircuit board comprises patterning a conductive layer of the printedcircuit board.
 34. The method according to claim 32, wherein thediaphragm comprises a conducting material, wherein the back platecomprises a conducting material, further comprising: a bias voltagebetween the diaphragm and the back plate, wherein flexing of thediaphragm causes a change in electric charge across the diaphragm andthe back plate, wherein the output signal is produced from the change inelectric charge across the diaphragm and the back plate.
 35. The methodaccording to claim 32, wherein the diaphragm comprises a permanentelectric charge, wherein the back plate comprises a conducting material,wherein flexing of the diaphragm causes a voltage change across thediaphragm and the back plate, wherein the output signal is produced fromthe voltage change across the diaphragm and the back plate.
 36. Themethod according to claim 32, wherein the diaphragm comprises aconducting material, wherein the back plate comprises a permanentelectric charge, wherein flexing of the diaphragm causes a voltagechange across the diaphragm and the back plate, wherein the outputsignal is produced from the voltage change across the diaphragm and theback plate.
 37. The method according to claim 32, wherein the firstvolume is in low acoustic impedance contact with the ambient acousticenvironment and the second volume is in high acoustic impedance contactwith the ambient acoustic environment.
 38. The method according to claim32, wherein the first volume is in high acoustic impedance contact withthe ambient acoustic environment the second volume is in low acousticimpedance contact with the ambient acoustic environment.
 39. The methodaccording to claim 37, further comprising: providing an enclosure,wherein the second volume is within the enclosure.
 40. The methodaccording to claim 38, further comprising: providing an enclosure,wherein the first volume is within the enclosure.
 41. The methodaccording to claim 39, wherein the enclosure comprises: a printedcircuit board; a bottom printed circuit board; one or more spacerprinted circuit boards, wherein the printed circuit board, the one ormore spacer printed circuit boards, and the bottom printed circuit boardare attached to form the enclosure.
 42. The method according to claim40, wherein the enclosure comprises: the printed circuit board; a bottomprinted circuit board; one or more spacer printed circuit boards,wherein the printed circuit board, the one or more spacer printedcircuit boards, and the bottom printed circuit board are attached toform the enclosure.
 43. The method according to claim 41, wherein thediaphragm is within the enclosure.
 44. The method according to claim 42,wherein the diaphragm is within the enclosure.
 45. The method accordingto claim 41, wherein the second volume is formed by the printed circuitboard, the diaphragm, the one or more spacer printed circuit boards, andthe bottom printed circuit board.
 46. The method according to claim 42,wherein the first volume is formed by the diaphragm and the back plate.47. The method according to claim 37, further comprising creating one ormore apertures through the printed circuit board that allow acousticwaves to pass through the one or more apertures from the ambientacoustic environment into the first volume and from the first volume tothe ambient acoustic environment.
 48. The method according to claim 38,further comprising creating one or more apertures through the printedcircuit board that allow acoustic waves to pass through the one or moreapertures from the ambient acoustic environment into the second volumeand from the second volume to the ambient acoustic environment.
 49. Themethod according to claim 34, further comprising applying a bias voltagebetween the back plate and the diaphragm.
 50. The method according toclaim 49, wherein applying a bias voltage between the back plate and thediaphragm comprises applying a bias voltage via interface electronics.51. The method according to claim 49, further comprising producing theoutput signal via an electrical impedance buffer amplifier.
 52. Themethod according to claim 51, wherein the output signal related to theacoustic pressure on the diaphragm is a low impedance output signal. 53.The method according to claim 32, wherein the diaphragm is micromachinedon the printed circuit board.
 54. The method according to claim 53,wherein the diaphragm is grown or deposited onto the printed circuitboard.
 55. The method according to claim 32, wherein the diaphragm isplaced on the printed circuit board.
 56. The method according to claim37, further comprising providing a vent passing through the enclosurethat allows ambient fluid to pass back and forth between the secondvolume and the ambient acoustic environment while maintaining the secondvolume in high acoustic impedance with the ambient acoustic environment.57. The method according to claim 56, wherein the ambient fluid is air.58. The method according to claim 38, further comprising providing avent passing through the enclosure that allows ambient fluid to passback and forth between the first volume and the ambient acousticenvironment while maintaining the first volume in high acousticimpedance with ambient acoustic environment.
 59. The method according toclaim 58, wherein the ambient fluid is air.
 60. The method according toclaim 37, further comprising: providing a vent to allow ambient fluid topass back and forth between the first volume and the second volume whilemaintaining the second volume in high acoustic impedance with theambient acoustic environment.
 61. The method according to claim 60,wherein the ambient fluid is air.
 62. The method according to claim 38,further comprising: providing a vent to allow ambient fluid to pass backand forth between the second volume and the first volume whilemaintaining the first volume in high acoustic impedance with the ambientacoustic environment.
 63. The microphone according to claim 62, whereinthe ambient fluid is air.